I’ve spent the past 30 minutes trying to reliably replicate this issue but I can’t find anything consistent. I’m getting content in Cards simply saying Unknown (see bottom of screen shot):
This is the card and you’ll see there’s nothing below the Button component. It’s on a global but despite it being replicated in four places, it can appear in different places on different instances.
It won’t be any help (I think) as it’s not showing up now. I saved when it happened but on opening it was back to normal. It’s happened twice but I still can’t reproduce it reliably.
Did you have an image that wasn’t loaded? If you look in your resources folder sometimes when opening a project the image has this symbol next to it when you try putting it in a container you will get same message. In early days this would happen but with upgrades this has gone away. Closing the project and Elements down and restarting project usually makes them “disappear”.